Connecting Chassis Metabolism with Glycosyltransferase Catalysis: The Role of Nucleotide Sugars in Human Milk Oligosaccharide Biosynthesis

Nucleotide sugars connect host metabolism to glycosyltransferase(GT)-catalyzed human milk oligosaccharide (HMO) biosynthesis, yet larger donor pools do not necessarily increase product formation. This review examines GDP-fucose, UDP-N-acetylglucosamine, UDP-galactose, and CMP-N-acetylneuraminic acid in relation to precursor competition, nucleotide regeneration, catalytic performance, and host physiology. It distinguishes direct donor consumption from upstream precursor, energy, biomass, and maintenance requirements and compares fermentation, whole-cell conversion, and cell-free biocatalysis. Published studies show that production depends on coordination among donor formation, active GT capacity, acceptor accessibility, competing metabolism, and process conditions. Sampling and extraction artifacts can distort donor-pool estimates, while in vitro kinetic parameters may not predict performance in cellular systems. Manufacturing assessment also requires attention to catalyst lifetime, product recovery, structural quality, and scale-up. An experimental framework is proposed to guide measurements and independent perturbations of donor supply, GT activity, acceptor access, and transport. HMO production should be assessed by yield, productivity, selectivity, structural identity, and stability, rather than donor abundance or endpoint titer alone.